Method and device for detecting insulation damage to a buried object

ABSTRACT

A method of and an apparatus for detecting damage to a buried object which includes the steps of (a) providing a buried object extending in a longitudinal direction which has an electrically insulating layer over a conductive member, (b) connecting one terminal of a power supply to the conductive member at a first position thereon, (c) connecting the other terminal of the power supply to an ammeter which is grounded, (d) connecting both terminals of at least one voltmeter to the conductive member on each side of the first position with the terminals of each voltmeter being spaced apart in the longitudinal direction, (e) connecting at least one voltmeter and the ammeter to data processing equipment for analyzing output values from the at least one voltmeter and the ammeter when power is supplied to the conductive member from the power supply, (f) supplying electric current from the power supply to the conductive member, and (g) measuring and comparing output values from the ammeter and the at least one voltmeter for detecting the presence of damage to the buried object and the relative location of any damage present as indicated by larger output values from any of the at least one voltmeter connected to the conductive member between the first position and a point of damage to the buried object.

This application is a continuation of now abandoned application Ser. No.687,420, filed Dec. 12, 1984, now abandoned.

TECHNICAL FIELD

The present invention relates to a method and apparatus for detectingdamage to a buried object covered with an electrically insulating layerover the outer periphery of a conductive member, such as a buried pipemember covered with a coating layer over the outer periphery of a steelpipe.

BACKGROUND OF THE INVENTION

In order to assure sound maintenance of pipes for transporting a fluid,it is essential to prevent surface corrosion as well as damage due toroadwork, etc.

For prevention of surface corrosion, a polyethylene lining for a layercovering the peripheral surface of a steel pipe, has recently beenemployed, thereby improving greatly the insulating performance orcorrosion proof characteristic. Additional provision of electricanticorrosion assures prevention of surface corrosion.

However, damage to a steel pipe due to roadwork etc. on the road surfacemay cause a leak to occur, resulting in an accident. In order to preventan accident, the managing agent of a buried object is required toperform extensive monitoring and management tasks such as by patrollingaround the buried object or by being present at roadwork performednearby the buried object. In this connection, immediate detection ofpipe damage caused for some reason may help and assist such management,and is very effective for avoiding an accident.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of rapidlydetecting damage to a buried object.

It is another object of the present invention to provide a method ofdetecting the damaged portion of a buried object.

The present invention provides a method of detecting damage to a buriedobject comprising the steps of flowing an electric current through theconductive member of the buried object covered with an electricallyinsulating layer over the outer periphery of the conductive member, andcomparing the value of such electric current with the value of anotherelectric current measured at a different time so as to detect damage tothe buried object.

The present invention also provides a method of detecting damage to aburied object comprising the steps of connecting one terminal of a powersupply to the conductive member of a longitudinally extending buriedobject covered with an electrically insulating layer over the outerperiphery of the conductive member, grounding the other terminal of thepower supply, and making a comparison between values of electriccurrents flowing at the same time through the buried object at bothsides in the longitudinal direction of the buried object with respect tothe portion of the conductive member connected to said one terminal ofthe power supply so as to detect damage to the buried object.

Furthermore the present invention provides a method of detecting damageto a buried object comprising the steps of connecting one terminal of apower supply to the conductive member of a longitudinally extendingburied object covered with an electrically insulating layer, groundingthe other terminal of the power supply, making a comparison betweenvalues of electric currents flowing at the same time through the buriedobject at both sides in the longitudinal direction of the buried objectwith respect to the portion of the conductive member connected to saidone terminal of the power supply, and measuring the size of damage tothe buried object based on soil specific resistance and a difference ofvalues of electric currents if such difference is detected.

The present invention still also provides a method of detecting damageto a buried object comprising the steps of connecting first terminals ofpower supplies to a first plurality of connection portions of theconductive member of a longitudinally extending buried object coveredwith an electrically insulating layer, grounding the other terminals ofthe power supplies, and making comparisons among values of electriccurrents flowing at the same time through the conductive member at asecond plurality of portions among said first plurality of connectionportions so as to detect damage to the buried object.

The present invention provides a device for detecting damage to a buriedobject covered with an electrically insulating layer over the outerperiphery of a conductive member, which comprises a power supply forflowing an electric current through the conductive member and means formaking comparisons among values of electric currents flowing atdifferent times.

The present invention also provides a device for detecting damage to alongitudinally extending buried object, which comprises (a) a powersupply having one terminal connected to the conductive member and theother terminal grounded, (b) a plurality of electric current detectormeans for detecting electric currents flowing through the conductivemember at both sides in the longitudinal direction of the buried objectwith respect to the portion of the conductive member connected to saidone terminal of the power supply, and (c) means responsive to outputsfrom the electric current detector means for making a comparison betweenvalues of electric currents detected at the same time.

The present invention still also provides a device for detecting damageto a longitudinally extending buried object covered with an electricallyinsulating layer over the outer periphery of a conductive membercomprising (d) a power supply having one terminal thereof connected tothe conductive member and the other terminal thereof grounded, (e) aplurality of electric current detector means for detecting electriccurrents flowing through the conductive member at both sides in thelongitudinal direction of the buried object with respect to the portionof the conductive member connected to said one terminal of the powersupply, (f) means responsive to outputs from the electric currentdetector means for making comparisons among values of electric currentsdetected at the same time, (g) means for detecting specific resistanceof soil, and (h) means responsive to the output values from the electriccurrent detector means, the electric current comparing means, and thesoil specific resistance detector means, for calculating and measuringthe size of damage based on soil specific resistance and a differencebetween values of electric currents if such difference is detected.

Furthermore the present invention provides a device for detecting damageto a longitudinally extending buried object covered with an electricallyinsulating layer over the outer periphery of a conductive membercomprising (i) power supplies positioned along the buried object at afirst plurality of locations and having first terminals thereofconnected to the conductive member and the other terminals thereofgrounded, (j) a second plurality of means for detecting electriccurrents flowing through the conductive member at the second pluralityof positions among said power-connected portions of the conductivemember, and (k) means responsive to outputs from the electric currentdetector means for making comparisons among values of electric currentsdetected at the same time.

According to the present invention, a comparison is made between thevalues of electric currents flowing at different times through aconductive member of a buried object. Therefore, damage to theelectrically insulating layer covering the conductive member may berapidly detected. According to the present invention, furthermore, apower supply is connected to a buried object at a portion of conductivemember thereof and a comparison is made between the values of electriccurrents flowing through the conductive member at both sides in thelongitudinal direction of the buried object with respect to thepower-connected portion. If damage is caused to the covering layer atone side with respect to the power-connected portion of the conductivemember, a larger amount of electric current flows through the conductivemember at its damaged side, thus detecting which side of the conductivemember with respect to its power-connected portion is damaged.

Moreover, the size of such damage may be calculated and measured baseson soil specific resistance and a difference between pipe electriccurrents if such difference is detected.

According to the present invention, power supplies are connected to aplurality of portions of the conductive member of a buried object spacedapart in the longitudinal direction of the buried object, andmeasurement and comparison are made in regard to the values of electriccurrents flowing at the same time through the conductive member at aplurality of portions among the power-connected portions of theconductive member. If such electric current values are different fromone another, this means that the buried object is being damaged. A quickdamage detection is thus effected. Moreover, it is understood thatdamage to a buried object is present between its portions of whichelectric current values are different from each other. Thus, the portionof damage is assertained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an underground sectional view of an embodiment of the presentinvention;

FIG. 2 is a cross section perpendicular to the axis of a pipe member 2;

FIG. 3 is a sectional view showing a state of a covering layer 4 damagedby an excavator 14 at a roadwork position 13;

FIG. 4 is a graph showing a change of electric current values with thepassage of time which are found by an ammeter 8 of FIG. 1;

FIG. 5 is an underground sectional view of another embodiment of theinvention;

FIG. 6 is a graph showing a change of the value of electric currentflowing through the ammeter 8 of FIG. 5;

FIG. 7 is an enlarged view showing a state of a voltmeter 22 connectedto a buried object 2;

FIG. 8 is a block diagram illustrating an electrical arrangement of theinvention;

FIG. 9 is a sectional view showing a system for detecting a damagedportion 32 of the pipe member 2;

FIG. 10 is a graph showing an electric potential gradient of the earthsurface along the longitudinal direction of the pipe member 2;

FIG. 11 is an underground sectional view of still another embodiment ofthe invention;

FIG. 12 is an enlarged view showing a state of a voltmeter 58 connectedto the buried object 2;

FIG. 13 is a waveform diagram of electric currents Ia and Ib;

FIG. 14 is a graph illustrating the relationship between values ofelectric currents Ia and Ib flowing at the same time; and

FIG. 15 is an enlarged sectional view showing the buried object 2 whenit is damaged at a roadwork position 13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a section view of an embodiment of the present invention.

A pipe member 2 for transporting a fluid is buried under the ground 1.The section of the pipe member 2 perpendicular to the axis thereof isshown in FIG. 2.

Applied to the exterior of a steel pipe 3 of the pipe member 2 is acovering layer 4 made of an electrically insulating material such aspolyethylene. This layer 4 covers the steel pipe 3 over its entireperiphery along its overall length.

One terminal of an anticorrosive power supply 7 is connected to theportion 6 of the pipe member 2. The other terminal of the power supply 7is grounded through an ammeter 8. Current values measured by the ammeter8 are set to a receiver 11 through a transmitter 9 and a transmissionline 10, and are displayed by a display device 12. An electric currentflows through the steel pipe 3 of the pipe member 2 from theanticorrosive power supply 7, thus providing an electric anticorrosiveeffect on the pipe member 2.

Where there is no damage to the covering layer 4, an anti-corrosiveelectric current on the order of, for example, a microampere is measuredby the ammeter 8. If small damage is present on the covering layer 4, anelectric current in the order of, for example, a milliampere is measuredby the ammeter 8.

It is assumed that a metallic excavator tool 14 inserted into the ground1 at a roadwork site 13, brakes the covering layer 4 and comes incontact with the steel pipe 3, as clearly shown in FIG. 3. In this case,not only an anticorrosive electric current but also an electric currentfrom the excavator 14 flow through the steel pipe 3. As shown in FIG. 4,a value of the electric current detected by the ammeter 8 is arelatively small value I₁ at an ordinary time before the excavator 14breaks the pipe member 2, but becomes a larger value I₂ after the timet1. This means that the pipe member 2 is damaged at the time t1.

Although the anticorrosive power supply 7 is employed in the embodimentdiscussed hereinbefore, there may be additionally disposed a detectionpower supply for flowing a larger electric current through the steelpipe 3 for detecting damage to the pipe member 2, and such detectionpower supply may be used only for periodical detection times, instead ofthe anticorrosive power supply 7.

FIG. 5 is a section view of another embodiment of the present inventionand like numerals are given to the corresponding parts.

In this embodiment, the respective first terminals of the anticorrosivepower supply 7 and a detection power supply 15 are connected to theportion 6 in the pipe member 2 of the steel pipe 3. The other terminalsof the power supplies 7 and 15 are respectively connected to thecontacts 17 and 18 of a changeover switch 16. The common contact 19 ofthe changeover switch 16 is grounded through the ammeter 8. Thechangeover switch 16 is actuated by relay circuit means comprising arelay circuit 47.

FIG. 6 shows the wave form of an electric current detected by an ammeter8. The common contact 19 of the changeover switch 16 is connected to thecontact 17 for a period W1, so that an anticorrosive electric currentflows through the steel pipe 23 for such period W1.

The common contact 19 of the changeover switch 16 is connected to thecontact 18 for a period W2 by the relay circuit 47, so that a largeelectric current for damage detection is supplied from the detectionpower supply 15. The period W1 is for example about 10 minutes, whilethe period time W2 may be, for example, 5 seconds. For the period W2,the detection power supply 15 supplies a large electric current to thesteel pipe 3 of the pipe member 2, thus facilitating detection of damageto the pipe member.

In FIG. 5, a plurality of voltmeters 22 and 23 are connected to thesteel pipe 3 of the pipe member 2 for measuring voltages correspondingto the electric currents flowing through the pipe member 2 at both sidesin the longitudinal direction thereof with respect to thepower-connected portion 6. As shown in FIG. 7, the terminals of thevoltmeter 22 are connected by leads to the steel pipe 3 of the pipemember 2 in longitudinally spaced relationship with each other.

An electric current Ia flowing through the steel pipe 3 is expressed bythe following equation: ##EQU1## where E=voltage measured by thevoltmeter 22

l=distance in meter between the portions 24 and 25 of the steel pipe 3connected to the voltmeters 22. Such distance is for example 30 m.

R=resistance per meter of the steel pipe 3

Likewise, the voltmeter 23 is connected to the steep pipe 3, therebymeasuring the voltage corresponding to the value of electric current Ibflowing through the steel pipe 3. One or a plurality of voltmeters 26,27, . . . may be additionally connected to the steel pipe 3 of the pipemember 2.

There are provided means for measuring soil specific resistance 42, 43,44, and 45 at the positions corresponding to the pipe electric currentmeasuring portions where the voltmeters 22, 23, 26 and 27 are disposed.There are also provided means for measuring pipe-ground electricpotential difference 62, 63, 64, and 65 corresponding to the voltmeters22, 23, 26, and 27.

FIG. 8 shows a block diagram showing an electric arrangement. Respectiveoutput values from the voltmeters 22, 23, 26, and 27, the ammeter 8, andthe soil specific resistance measuring means 42, 43, 44, and 45 areconverted into digital values by an analog-to-digital converter 48, andthen sent to a data processing circuit 49 comprising a microcomputer.The data processing circuit 49 samples and measures voltages, electriccurrents, and soil specific resistances at a plurality of times for theperiod W2, so as to calculate and measure the average, and maximum andminimum values of those respective values. Output data from the dataprocessing circuit 49 are transmitted to a remote supervisory controldevice 50.

This remote supervisory control device 50 comprises telemeter branchstation 51 for transmitting output data from the data processing circuit49, a transmission line 52, and a telemeter attended station 53 forperforming a control operation. A signal from the telemeter attendedstation 53 causes the telemeter branch station 51 to switch thechangeover switch 16 through the relay circuit 47.

The reference numerals 48, 49, 51, and 52 designate the respectivemembers in the gross with the additive small letters a, b, c, d, and eomitted.

It is assumed that the metallic excavator 14 inserted into the ground 1at the roadwork site 13, breaks the covering layer 4 of the pipe member2 and comes in contact with the steel pipe 3. In this case, a largeelectric current flows through the steel pipe 3 from the excavator 14.Such electric current exhibits a large value between the excavator 14and the power-connected portion 6 of the pipe member 2, while anelectric current flowing through the other portions of the steel pipe 3is small. That is, the values of electric currents Ib flowing throughthe steel pipe 3 corresponding to the voltages measured at the same timeby the voltmeters 23 and 26 are large and equal to each other. On theother hand, the value of an electric current Ia flowing through thesteel pipe 3 corresponding to the voltages measured by the voltmeter 22is small and the value of an electric current flowing through the steelpipe 3 corresponding to the voltages measured by the voltmeter 27 isalso small. When a comparison is made between the electric currents Iaand Ib flowing through the steel pipe 3 corresponding to the voltagemeasured by the voltmeters 22 and 23 which are disposed at both sides ofthe pipe member 2 in the longitudinal direction thereof with respect tothe power-connected portion 6, the value of the electric current Ibflowing through the steel pipe 3 corresponding to the voltage measuredby the voltmeter 23 is larger than Ia. This means that the pipe member 2is being damaged at the right side in FIG. 5 or at the side of thevoltmeter 23 which has detected the voltage corresponding to the largerelectric current, with respect to the power-connected portion 6.Moreover, since the electric current values corresponding to voltagesmeasured by the voltmeters 26 and 27 are different from each other, itis readily understood that the damage is present on the pipe betweenthese voltmeters 26 and 27.

It is assumed that the pipe-ground electric potential difference on theground corresponding to the damaged portion 14a of the pipe member 2 isindicated as V and the damage shape of the damaged portion 14a is acircle having a radius r. Where the damage radius r is sufficientlylarger than the covering layer thickness t, the damage radius r or thedamage size is approximated based on the following equation: ##EQU2##where ρ=correction coefficient with respect to soil specific resistanceof the damaged portion 14a (units =Ω·m)

i=electric current flowing through the steel pipe 3 at the damagedportion 14a (units=A)

V=the average of pipe-ground electrical potential differences on theground at both sides of the damaged portion 14a (units =V)

k=proportional constant (no units)

Although V undergoes a change with the passage of time, the average of Vfor the period W2 is substantially constant regardless of time.Therefore, pipe-ground electric potential differences V1 and V2 aremeasured at suitable time intervals by measuring means 64 and 65disposed at the pipe electric current measuring points at both sideswith respect to the damaged portion 14a, and the average of V1 and V2 isused as V for the equation 2. Such calculation is made by the telemeterattended station 53. The period W2 during which such sampling is made, adetermined to be for example about 10 minutes.

ρ varies with the place and the weather. Where the pipe electric currentmeasuring distance is determined to be a suitable one, for exampleseveral kilometers, ρ varies with only the weather. If, therefore, soilspecific resistance is measured at each of the pipe electric currentmeasuring points, the correction coefficient with respect to soilspecific resistance ρ of the pipe member 2 at a given portion isrepresented by the average of correction coefficients with respect tosoil specific resistance values ρ1 and ρ2 measured at the pipe electriccurrent measuring points on both sides with respect to said givenportion. Such calculation is made by the telemeter attended station 53.

The electric current i is found from the following equation:

    i=I26-I27                                                  (3)

where

I26=electric current flowing through the steel pipe 3 corresponding tothe voltage measued by the voltmeter 26 at one side with respect to thedamaged portion 14a

I27=electric current flowing through the steel pipe 3 corresponding tothe voltage measured by the voltmeter 27 at the other side with respectto the damaged portion 14a

The telemeter attended station 53 compares the values of pipe electriccurrents corresponding to the voltages measured at the substantiallysame time by the voltmeters 22, 23, 26, and 27 with the respectiveadjacent ones, based on the results measured for respective periods W2by the data processing circuit 49. As the consequence of suchcomparison, if the pipe electric currents I26 and I27 corresponding tothe voltages measured by the voltmeters 26 and 27 are different fromeach other, the size of damage is calculated, measured, and displayedbased on the difference i between the electric current values I26 andI27.

Thus calculated damage size depends on the contact area of the excavator14 with the ground and is larger than the real size of the damage of thepipe member 2, if the excavator 14 remains in contact with the steelpipe 3 and is electrically connected thereto. However, such contact ofthe excavator 14 with the steel pipe 3 is generally made for a shortperiod of time, for example such as a moment. In this connection, thecalculated damage size may be regarded as a value corresponding to thereal size of the damage of the pipe member 2, for practical purposes, inview of the fact that the electric current value i is averaged with thepassage of time.

FIG. 9 is a section view for precisely detecting the damage position ofthe pipe member 2. This embodiment is similar to the previousembodiment, and like reference numerals are given to the correspondingparts.

A plurality of voltmeters 28, 29, 30, and 31 provided on the surface ofthe earth or ground 1 at a plurality of positions in the longitudinaldirection of the pipe member 2. Both the terminals of each of thesevoltmeters 28, 29, 30, and 31 are inserted into the ground 1 atdifferent portions spaced apart along the longitudinal direction of thepipe member 2. An electric current is supplied to the steel pipe 3 ofthe pipe member 2 from either the anticorrosive power supply 7 or thedetection power supply 15, by the changeover switch 16 performing thechangeover operation.

If the covering layer 4 is damaged at a portion 32 due to roadwork orthe like, the voltages detected by the voltmeters 28, 29, 30, and 31become opposite in polarity on both sides in the longitudinal directionof the pipe member 2 with respect to the damaged portion 32 of the pipemember 2, as shown by arrows 33 and 34 in FIG. 9. Electric potentialdifference or earth surface electric potential gradient per unitdistance of the pipe member 2 in its longitudinal direction measured bythe voltmeters 28, 29, 30, and 31, is as shown in FIG. 10. That is, theelectric potential gradient becomes extremely small at the damagedportion 32. This means that the covering layer 4 is damaged at a portionexhibiting the small electric potential gradient.

In this embodiment also, a larger electric current is supplied to thesteel pipe 3 from the detection power supply 15, thus facilitatingmeasurement utilizing the voltmeters 28, 29, 30, and 31.

The present invention is not limited in application to the buried pipemember 2 discussed hereinbefore, but may be widely applied to buriedobjects covered with electrically insulating layers over the outerperipheries of conductive members.

The detection power supply 15 used for detecting damage to a buriedobject may be an AC power supply. In such case, both the terminals ofeach of the voltmeters 28, 29, 30, and 31 are not necessarily insertedinto the ground 1. Search coils may be disposed adjacent to the earthsurface, and AC voltages induced by the coils may be measured.

FIG. 11 is a sectional view of still another embodiment of the presentinvention.

A pipe member 2 for transporting a fluid is buried under the ground 1.There is connected to portions 6a and 6b of the pipe member 2 oneterminal of anticorrosive power supplies 7a and 7b, respectively, ofwhich the other terminals are grounded.

There are provided voltmeters 58 and 59 for measuring voltagescorresponding to the values of electric currents Ia and Ib at the sametime and comparing with each other through the steel pipe 3 at aplurality of positions (2 positions in this embodiment) among thepower-connected portions 6a and 6b.

As shown in FIG. 12, the terminals 58a and 58b of the voltmeter 58 areconnected to the steel pipe 3 in longitudinally spaced relationship witheach other.

An electric current Ia flowing through the steel pipe 3 of the pipemember 2, is expressed by the following equation: ##EQU3## whereE=voltage measured by the voltmeter 58

l=distance in meters between the connection portions 58a and 58b

R=resistance per meter of the steel pipe 3.

Likewise, the voltmeter 59 is connected to the steel pipe 3, thusmeasuring the voltage corresponding to an electric current Ib flowingthrough the steel pipe 3.

Where the covering layer 4 of the pipe member 2 is not damaged, thevalues of voltages corresponding to electric currents flowing throughthe steel pipe 3 measured by the voltmeters 58 and 59 are equal to eachother, and undergo a change with the passage of time, for example, asshown in FIG. 13. When these electric currents Ia and Ib are shown inrectangular coordinates, a straight line 60 having a gradient of 45° isfound as shown in FIG. 14.

It is assumed that the metallic excavator 14 inserted in the ground 1 atthe roadwork site 13 between the voltmeters 58 and 59 breaks thecovering layer 4 of the pipe member 2 and makes contact with the steelpipe 3. In this case, a large electric current flows through the steelpipe 3 from the excavator 14. Such electric current branches off atdifferent rates to the right- and left-sides of the steel pipe 3 withrespect to the roadwork site 13 in FIGS. 11 and 15. Therefore the valuesof electric currents Ia and Ib corresponding to voltages found at thesame time by the voltmeters 58 and 59, are different from each other. Asshown in FIG. 14, a set of measured electric currents (Ia, Ib) isdistrubuted on a line 61 moved in parallel from the line 60 passingthrough the origin O. Thus it is understood that the pipe member 2 isdamaged between its portions connected to the voltmeters 58 and 59. Thedistance between the voltmeters 58 and 59 may be, for example, 2 to 15km.

In this embodiment, anticorrosive electric currents flowing through thesteel pipe 3 of the pipe member 2 interact with each other in view ofthe fact that a plurality of power supplies 7a and 7b are disposed, andsuch electric currents are greatly affected by stray electric currentsin the ground. Thus, the state of such electric currents becomes asshown in FIG. 13. In this connection, it is difficult to detect whetheror not the covering layer 4 is damaged, by discriminating the respectivelevels of the current values Ia and Ib. However, in the above-mentionedembodiment, an accurate detection of damage to the covering layer 4 maybe executed even if the electric currents Ia and Ib detected at the sametime are greatly changed.

According to yet another embodiment of the present invention, damagedetection power supplies for supplying greater electric currents thanthose of the anticorrosive power supplies 7a and 7b, are respectivelyprovided for the power supplies 7a and 7b. Such damage detection powersupplies are periodically switched over to the power supplies 7a and 7b,thus facilitating measurement utilizing the voltmeters 58 and 59.

The present invention is not limited in application to the buried pipemember 2, but may be widely applied to longitudinally extending buriedobjects covered with electrically insulating layers over the outerperipheries of conductive members.

What is claimed is:
 1. A method of detecting damage to a buried objectcomprising the steps of:providing a buried object extending in alongitudinal direction, said object having an electrically insulatinglayer over a conductive member; connecting one terminal of a powersupply to said conductive member at a first position thereon; connectingthe other terminal of said power supply of an ammeter which is grounded;supplying electric current from said power supply to said conductivemember; and sending signals from said ammeter to a display device toindicate damage to said buried object when electric current measured bysaid ammeter increases from a value which corresponds to a value ofelectric current measured by said ammeter when said object is notdamaged.
 2. A method of detecting damage to a buried object comprisingthe steps of:providing a buried object extending in a longitudinaldirection, said object having an electrically insulating layer over aconductive member; connecting one terminal of a power supply to saidconductive member at a first position thereon; connecting the otherterminal of said power supply to an ammeter which is grounded;connecting both terminals of at least one first voltmeter to saidconductive member on one side of said first position and connecting bothterminals of at least one second voltmeter to said conductive member onthe other side of said first position with said terminals of said atleast one first voltmeter and said terminals of said at least one secondvoltmeter being spaced apart in said longitudinal direction; connectingsaid at least one first voltmeter, said at least one second voltmeterand said ammeter to data processing means for analyzing output valuesfrom said at least one first voltmeter, said at least one secondvoltmeter and said ammeter when power is supplied to said conductivemember from said power supply; supplying electric current from saidpower supply to said conductive member; and measuring and comparingoutput values from said ammeter, said at least one first voltmeter andsaid at least one second voltmeter for detecting the presence of damageto said buried object and the relative location of any damage present asindicated by larger output values from said at least one first voltmeteror said at least one second voltmeter connected to said conductivemember between said first position and a point of damage of said buriedobject.
 3. A method of detecting damage to a buried object comprisingthe steps of:providing a buried object extending in a longitudinaldirection, said object having an electrically insulating layer over aconductive member; connecting one terminal of a power supply to saidconductive member at a first position thereon; connecting the otherterminal of said power supply to an ammeter which is grounded;connecting both terminals of at least one first voltmeter to saidconductive member on one side of said first position and connecting bothterminals of at least one second voltmeter to said conductive member onthe other side of said first position with said terminals of said atleast one first voltmeter and said terminals of said at least one secondvoltmeter being spaced apart in said longitudinal direction; connectingsoil specific resistance measuring means to soil located at eachposition corresponding to said at least one first voltmeter and said atleast one second voltmeter; connecting said at least one firstvoltmeter, said at least one second voltmeter, said soil specificmeasuring means and said ammeter to data processing means for analyzingoutput values from said at least one first voltmeter, said at least onesecond voltmeter and said ammeter when power is supplied to saidconductive member from said power supply; supplying electric currentfrom said power supply to said conductive member; and measuring andcomparing output values from said ammeter, said soil specific resistancemeasuring means, said at least one first voltmeter and said at least onesecond voltmeter for detecting the presence of damage to said buriedobject and the relative location of any damage present as indicated bylarger output values from said at least one first voltmeter or said atleast one second voltmeter connected to said conductive member betweensaid first position and a point of damage of said buried object.
 4. Amethod of detecting damage to a buried object comprising the stepsof:providing a buried object extending in a longitudinal direction, saidobject having an electrically insulating layer over a conductive member;connecting one terminal of a power supply to said conductive member at afirst position thereon; connecting the other terminal of said powersupply to an ammeter which is grounded; connecting both terminals of atleast one first voltmeter to said conductive member on one side of saidfirst position and connecting both terminals of at least one secondvoltmeter to said conductive member on the other side of said firstposition with said terminals of said at least one first voltmeter andsaid terminals of said at least one second voltmeter being spaced apartin said longitudinal direction; connecting soil specific resistancemeasuring means to soil located at each position corresponding to saidat least one first voltmeter and said at least one second voltmeter;connecting pipe-ground electric potential difference measuring means ateach position corresponding to said at least one first voltmeter andsaid at least one second voltmeter; connecting said at least one firstvoltmeter, said at least one second voltmeter, said soil specificresistance measuring means, said pipe-ground electric potentialdifference measuring means and said ammeter to data processing means foranalyzing output values from said at least one first voltmeter, said atleast one second voltmeter and said ammeter when power is supplied tosaid conductive member from said power supply; supplying electriccurrent from said power supply to said conductive member; and measuringand comparing output values from said ammeter, said soil specificresistance measuring means, said pipe-ground electric potentialdifference measuring means, said at least one first voltmeter and saidat least one second voltmeter for detecting the presence of damage tosaid buried object and the relative location of any damage present asindicated by larger output values from said at least one first voltmeteror said at least one second voltmeter connected to said conductivemember between said first position and a point of damage of said buriedobject.
 5. A device for detecting damage to a buried object whichincludes an electrically insulating layer over the outer periphery of aconductive member, comprising:an anticorrosive power supply having apair of terminals, one of said terminals being connected to a portion ofsaid conductive member; a detection power supply having a pair ofterminals, one of said terminals being connected to said portion of saidconductive member; a changeover switch having one contact connected tothe other one of said terminals of said anticorrosive power supply andsaid changeover switch having another contact connected to the other oneof said terminals of said detection power supply, said one contact andsaid other contact being selectively connected to a common contact ofsaid changeover switch; relay circuit means connected to said changeoverswitch for selectively connecting said one contact or said other contactto said common contact; an ammeter connected to said common contact ofsaid changeover switch; a plurality of voltmeters, each of saidvoltmeters having a pair of terminals connected to said conductivemember with each terminal being spaced apart along the length of saidconductive member, at least one of said voltmeters being connected tosaid conductive member on either side of said portion of said conductivemember; a plurality of means for measuring soil specific resistance eachof which is located at a position corresponding to a respective one ofsaid plurality of voltmeters; a plurality of means for measuringpipe-ground electric potential difference each of which is located at aposition corresponding to a respective one of said plurality ofvoltmeters; a plurality of analog to digital converters each of which isconnected to a respective one of said voltmeters and said ammeter, arespective one of said means for measuring soil specific resistance anda respective one of said means for measuring pipe-ground electricpotential difference; a plurality of data processing circuits each ofwhich is connected to a respective one of said plurality of analog todigital converters; a remote supervisory control device connected toeach of said plurality of data processing circuits for controllingoperation of said changeover switch and analyzing data from said dataprocessing circuits to determine the presence of damage to said buriedobject and the extent of any damage detected.